Method and an apparatus for analyzing trace impurities in gases

ABSTRACT

The present invention relates to a method and an apparatus for analyzing trace impurities in gases, which enable to analyze a very small quantity of impurities by only a simple operation, without making the column arrangement or the structure of flow complicated. In the apparatus and the method, when the trace impurities are measured in ppb-sub ppb level by a combined analyzer which is equipped with an atmospheric pressure ionization mass spectrometer to the back of a gas chromatography, a mixed gas of various gases is used as a carrier gas or purified gases added to the gases outflowed from a gas chromatography.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method and an apparatus foranalyzing trace impurities in gases, and particularly to a method and anapparatus for detecting ppb-sub ppb level of trace impurities in varioushigh-purified gases, by a combined analyzer comprised of gaschromatography and atmospheric pressure ionization mass spectrometer.

[0003] 2. Description of the Prior Art

[0004] In case of analyzing (detecting) the impurities in high-purifiedgases by combined analyzer comprised of gas chromatography andatmospheric pressure ionization mass spectrometer, the outflowedgas(30-50 cc/min) from a gas chromatography which uses packed column,does not reach the gas flow(100-500 cc/min) necessary for a commonatmospheric pressure ionization mass spectrometer. Therefore, theoutflowed gas from a gas chromatography was added by purified gases, thekind of which is the same with the carrier gas used in the gaschromatography, and then introduced into an atmospheric pressureionization mass spectrometer(Japanese Patent Laid Open Gazette Hei.6-3471, Japanese Patent Laid Open Gazette Hei.9-15207).

[0005] High-purified He(helium) or Ar gas is commonly used as thecarrier gas and the purified gas. In particular, He is much preferred,because its ionization potential (24.59 eV) is higher than those ofother gases, and all kinds of impurities except He can be detected.

[0006] Nevertheless, when He gas is used as a carrier gas, it is alsosensitive to some main component such as O₂, N₂ and Ar, and their ionsare generated in a large quantities. Therefore, some impurities hardlyseparable from the main components could not be detected well, and themeasurement with a high sensitivity was hard to be obtained.

[0007] Under the circumstance, a method, in which impurities and maincomponents are separated before being introduced into the atmosphericpressure ionization mass spectrometer, has been carried out to improvedthe sensitivity of measurement. For example, a complicated applied flowpassage using a method such as “Heart Cut method”, is installed and thencut just before the main component is outflowed from a gaschromatography, and the outflowed gas is introduced into the atmosphericpressure ionization mass spectrometer about at the point of time whenthe outflow of the main component ends.

[0008] However, there is a problem in the above method, in that the maincomponents near the impurities were not separated well, and theintroduction of the main components into the atmospheric pressureionization mass spectrometer could not be avoided completely, whichcauses increased noises and weakened sensitivity when analyzing theimpurities.

[0009] There is another problem that as columns used for a long-termcould increase the retention time of the main components and thus thesubject impurities could be separated and removed. Therefore, theretention time should be checked regularly, which requires much labors,and many columns for removing the main components and complicated floware also required.

[0010] There is still another problem in another method of removing themain components by an absorbent, in that the subject impurities areremoved together with the main components, or the other impurities aregenerated from the absorbent in ppb-ppm levels, resulting that themeasurement in ppb level is hard to perform.

[0011] In addition, in case of using a atmospheric pressure ionizationmass spectrometer, as He generates helium cluster ion (He₄+)(massnumber=16) in a large amount, CH₃+(mass number=15) is measured foranalyzing methane(M.W.=16). However, there is a problem that thesensitivity of mass number 15 is not so good as mass number 16(CH₄+).Moreover, as the discharge of He is less stable compared with othergases and the stability of the main components are not good, thesensitivity to H₂, which is detected by He₂H+(mass number=9) generatedby combining He₂+ and proton, is also bad.

[0012] In case of Ar, which has a good discharge nature, there is aproblem that the ionization potential of Ar(15.76 eV) is near that of N₂(15.58 eV), and thus transfer of electric charges is hardly generatedand the measurement of the impurities like N₂ or Ne, whose ionizationpotentials are higher than those of Ar, is impossible.

[0013] Under the circumstance, when the impurities are measured to thelevel of sub ppb in high purified O₂, He is used for Ne or N₂ ofimpurities, Ar is used for of H₂, CH₄, CO or CO₂ of impurities. That is,He should be supplied for analyzing N₂ etc., and Ar should be suppliedfor analyzing H₂ etc., as a carrier gas and purified gases added.Therefore, much labor and time were required for switching the supply ofthe gases, and the measurement cannot be carried out quickly.

[0014] The other method has been proposed, in which a third componentgas is incorporated into the sample gas when the analysis is performedby atmospheric pressure ionization mass spectrometer alone (JapanesePatent Laid-Open Gazette Hei.6-74940). However, the method has a problemthat the impurities in the third component gas should be checked inadvance, because they could have an effect on the analysis of theimpurities in the sample gas.

[0015] For example, when analyzing N₂ as an impurity in Ar, as theionization potentials of both gases are mutually near each other, thesensitivity of the analysis is very bad and the measurement cannot becarried out at ppb level. Therefore, a method has been proposed, inwhich the measurement is performed by an atmospheric pressure ionizationmass spectrometer using proton transfer rection after adding H₂ in thelevel of % into the sample gas of Ar. In this method, the check of theimpurity N₂ in H₂ added, or the separation of CO in Ar both of whichhave the same mass number is hardly performed. Therefore, themeasurement is carried out for N₂+CO, and thus the concentration of COin the sample gas should be checked in advance, resulting in a verytedious and bothering work.

SUMMARY OF THE INVENTION

[0016] The present invention is purposed to provide a method and anapparatus for analyzing trace impurities in gases, which enable toanalyze a very small quantity of impurities by a simple operation,without making the column arrangement or the structure of flowcomplicated.

[0017] In accordance with the object of the present invention, there isprovided a method for analyzing the trace impurities in gases, themethod comprising the steps of:

[0018] separating main components and trace impurities from a sample gasconveyed by a carrier gas by gas chromatography;

[0019] introducing the gases outflowed from said gas chromatography intoan atmospheric pressure ionization mass spectrometer; and

[0020] analyzing the trace impurities, wherein a mixed gas is used asthe carrier gas.

[0021] In accordance with another object of the present invention, thereis provided a method of analyzing the trace impurities in gases, themethod comprising the steps of:

[0022] separating main components and trace impurities from a sample gasconveyed by a carrier gas by gas chromatography;

[0023] adding purified gas into the gases outflowed from said gaschromatography; introducing the gases into an atmospheric pressureionization mass spectrometer; and

[0024] analyzing the trace impurities, wherein a mixed gas is used as atleast one of the carrier gas and the purified gas.

[0025] In accordance with still another object of the present invention,there is provided a method of analyzing the trace impurities in gases,the method comprising the steps of:

[0026] separating main components and trace impurities from a sample gasconveyed by a carrier gas by gas chromatography;

[0027] introducing the gases outflowed from said gas chromatography intoan atmospheric pressure ionization mass spectrometer; and

[0028] analyzing the trace impurities, wherein a single component gas isused as the carrier gas, and purified gas which are different from thecarrier gas, are added to the outflowed gas.

[0029] In the above case, the method is characterized in that if saidcarrier gas is He, the added purified gas is Ar alone, or He—Ar mixedgas, or He—H₂ or Ar—H₂ mixed gas, and if said carrier gas is Ar, theadded purified gas is He alone, or He—Ar mixed gas.

[0030] In accordance with still another object of the present invention,there is provided a method of analyzing the trace impurities in gases,the method comprising the steps of:

[0031] introducing a sample gas into a gas chromatography using He as acarrier gas;

[0032] separating main components and trace impurities from the samplegas;

[0033] adding a purified gas of Ar alone or Ar—He mixed gas into thegases outflowed from the gas chromatography;

[0034] introducing the gases into an atmospheric pressure ionizationmass spectrometer; and

[0035] analyzing the trace impurities, wherein the impurity of H₂ isdetected by mass number 41 or 81, and the impurity of methane isdetected by mass number 16.

[0036] In accordance with still further object of the present invention,there is provided a method of analyzing the trace impurities in gases,the method comprising the steps of:

[0037] introducing a sample gas into a gas chromatography using He as acarrier gas;

[0038] separating main components and trace impurities from the samplegas;

[0039] adding a purified gas into the gases outflowed from the gaschromatography;

[0040] introducing the gases into an atmospheric pressure ionizationmass spectrometer; and

[0041] analyzing the trace impurities, wherein at least two purifiedgases selected from i) He alone, ii) He—Ar mixed gas and iii) He—H₂mixed gas or Ar-H₂ mixed gas, are selected and switchably used as saidpurified gas.

[0042] In accordance with still further object of the present invention,there is provided an apparatus for analyzing the trace impurities ingas, the apparatus comprising:

[0043] a gas chromatography for separating the main component and traceimpurities from the sample gas conveyed by carrier gas;

[0044] an atmospheric pressure ionization mass spectrometer connected tothe back part of the gas chromatography; and

[0045] a purified gas adding passage for adding the purified gasoutflowed from the gas chromatography, the purified gas adding passagebeing installed in a passage between the gas escaping part of the gaschromatography and the gas introduction passage of the atmosphericpressure ionization mass spectrometer,

[0046] wherein i) a passage for supplying a purified gas whose kind isthe same as the carrier gas, ii) a passage for supplying a purified gaswhose kind is different from the carrier gas, and iii) mixed ratioregulating means installed in the purified gas adding passage forregulating a mixed ratio of both purified gases, and added amountcontrolling means installed in the purified gas adding passage forregulating the added amount of purified gases depending on the kind ofthe outflowed gas are further provided.

BRIEF DESCRIPTION OF THE DRAWING

[0047] For fuller understanding of the nature and objects of theinvention, reference should be made to the following detaileddescription taken in conjunction with the accompanying drawing in which”

[0048]FIG. 1 is a flow diagram for showing a first example of thepresent apparatus.

[0049]FIG. 2 is a flow diagram for showing a second example of thepresent apparatus.

[0050]FIG. 3 is a flow diagram for showing a third example of thepresent apparatus.

[0051]FIG. 4 is a flow diagram for showing a fourth example of thepresent apparatus.

[0052]FIG. 5 is a diagram for showing the relation between theconcentration of Ar and the peak intensity of the respective impuritiesin the first example.

[0053]FIG. 6 is a drawing for comparing the peaks of methane accordingto the existence of Ar of the first example.

[0054]FIG. 7 is a diagram for showing the calibration curve, in the caseof adding Ar of the first example.

[0055]FIG. 8 is a drawing for comparing the peaks of CO according to theexistence of Ar of the second example.

[0056]FIG. 9 is a diagram for showing the calibration curve of CO, inthe case of adding Ar of the second example.

[0057]FIG. 10 is a drawing for comparing the peaks of CO according tothe existence of He of the third example.

[0058]FIG. 11 is a diagram for comparing the peaks of N₂, in the case ofadding H₂ of the fifth example.

[0059]FIG. 12 is a diagram for showing the calibration curve of N₂, inthe case of adding H₂ of the fifth example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060]FIG. 1 is a flow diagram for showing a first example of thepresent apparatus.

[0061] The apparatus is provided with i) a main purified gas supplysystem (30) for supplying a main purified gas for use as a purified gasto be added to the carrier gas of the gas chromatography(10) and a gasoutflowed from the gas chromatography(10),

[0062] ii) a sub-purified gas supply system(31) for supplying asub-purified gas to be mixed with the main purified gas and then addedto the outflowed gas, and

[0063] iii) a purified gas supply regulating means(32) for regulatingthe supplied amount of the main purified gas and sub-purified gas to beadded into the outflowed gas, together with iv) an atmospheric pressureionization mass spectrometer(11) as a detecting part of a gaschromatography(10), which is installed in the back of the gaschromatography(10), for separating the main components and traceimpurities from the sample gas.

[0064] The gas chromatography(10), formed by connecting i) a separatingcolumn(13) packed with a separator through an 8 way gas switchingcock(12), ii) two sample loop(14 a, 14 b), iii) a sample gasintroduction passage(15) connected to the sample gas source, iv) acarrier gas introduction passage(16) connected to the main purified gassupply system(30) and an exhaust passage(20) provided with a pressuregauge(17), a back pressure regulator(18) and a mass flow meter(19),serves to introduce the sample gas metered by a sample loop(14 a,14 b)into the separating column(13) by operating the 8 way gas switchingcock(11), and to separate the respective components of the sample gas inthe separating column(13) so that the components may be moved in orderinto the separated gas outflowing passage(21).

[0065] The atmospheric pressure ionization mass spectrometer(11) has anion source part(22) and a mass separating part/detecting part(23), theion source part(22) being connected with a gas introduction passageconnected with the separated gas outflowing passage(21) and a gasescaping passage(25) for discharging the surplus gases. Further, the gasescaping passage(25) is provided with a pressure gauge(26) formaintaining the pressure of the ion source part(22), a back pressureregulator(27) and a mass flow meter(28).

[0066] The main purified gas supply system(30) and the sub-purified gassupply system(31) are provided with pressure regulators(33,34) andpurifier(35,36) respectively. The main purified gas supply system(30) isdiverged in two parts in the lower of the purifier(35), a passage(37) ofthe two parts is connected to the carrier gas introduction passage(16)of the gas chromatography(10) through the mass flow controller(38),whereas the other passage(39) of the two parts is connected to thepurified gas supply regulating means(32). In addition, in thesub-purified gas supply system(31), the back of the purifier(36) isconnected to the purified gas supply regulating means(32).

[0067] The purified gas supply regulating means(32), for mixing the mainpurified gas from the passage(39) and the sub-purified gas from thesub-purified gas supply system(31) in a fixed ratio, and for regulatingthe added amount into the outflowed gas, is provided with a mass flowcontroller for main purified gas(40) and a mass flow controller for subpurified gas(41), as well as a supply valve for controlling the supplyof the sub-purified gas and a exhaust valve. The passage for dischargingthe purified gas after mixing(gas adding passage)(44) is connected inthe way of the passage from the separated gas outflowing passage(21) tothe gas introduction passage(24).

[0068] In addition, if an automatic controller(program controller) forcontrolling the flowing amount of the two mass flow controller and theopening/shutting of the valves(42,43) at the purified gas supplyregulating means(32), the regulation of the mixed ratio of the purifiedgas or the added amount of the gas could be carried out automatically byconnecting the controller with the operation of the gaschromatography(10).

[0069] As the purified gas and sub-purified gas, a single component gassuch as He, Ar, N₂ and H₂, or a mixed gas such as He—Ar mixed gas, He—H₂mixed gas, and Ar—H₂ mixed gas may be used. As a filler filled in theseparating column, various filler such as molecular sieve type or Unibeads type can be used.

[0070] The following is an example of the processing to analyze traceimpurities in a sample gas by using the above apparatus.

[0071] First, a mass flow controller or a back pressure regulator is setto a fixed value and the flow of the gases or the pressure in the systemis set according to a required condition for the analysis. The samplegas is introduced from the sample gas introduction passage(15), andflowed into one of the sample loop (14 a, 14 b), for example the sampleloop(14 a), through the 8 way gas switching cock(12).

[0072] Then, if the 8 way gas switching cock(12) is operated so as tomake the carrier gas flow into the sample loop(14 a) in which the samplegas flows, the fixed amount of the sample gas metered in the sampleloop(14 a) is incorporated into the carrier gas and introduced into theseparating column(13) where the components of the gas are separated inthe dispersant as they proceed into the separating column(13), andoutflowed from the separated gas outflowing passage(21) in apredetermined order.

[0073] The purified gas introduced from the gas adding passage(44) isadded into the gas outflowed to the separated gas outflowing passage(theoutflowed gas of the gas chromatography) in a fixed amount, andintroduced into the ion source part(22) of the atmospheric pressureionization mass spectrometer(11) through the gas introductionpassage(24). The ion source part(22) is maintained to a predeterminedpressure, such as 0.4 kg/cm²(0.04 Mpa) by the back pressureregulator(27). Then, a part of the ionized gas after being introducedinto the ion source part(22) passes through a slit and is introducedinto the mass separating part/detecting part(23), and the ion current ofthe respective components is detected after the separation of each mass.

[0074] As explained in the above, the impurities in the sample gas canbe analyzed in a very sensitive and accurate way, by properly selectingthe main purified gas and the sub-purified gas, and setting the addedamount and the mixed ratio into the outflowed gas of the gaschromatography. In addition, as the components are separated in advancein the gas chromatography(10), even in the case that the added purifiedgases contain impurities, the kind of which is the same with the subjectimpurities to be analyzed, the peaks of the impurities in the sample gascan be clearly detected, and thus no prior check of the impurities inthe purified gas is required, contrary to the conventional method inwhich only the atmospheric pressure ionization mass spectrometer isused.

[0075] The composition(mixed ratio) or the added amount of the purifiedgas to be added to the outflowed gas, can be set according to the maincomponents of the sample gas, the impurities to be analyzed, the kindsof the main purified gas and the sub-purified gas and the like. In theabove example, the mixed ratio can be selected by properly setting theflowing amount from both the mass flow controllers(40,41). Further, onlythe main purified gas may be added into the outflowed gas in apredetermined amount, by shutting off the supply valve(42) and openingthe exhaust valve(43).

[0076] As above, the effect by the main components of the sample gas canbe removed by adding a sub-purified gas different from the carrier gas.For example, in case the main purified gas is He, the sub-purified gasis Ar, and the main components of the sample gas are not only Ar butalso N₂, almost all the N₂ in the main components would be removed byadding He—Ar mixed gas. In addition, in case the main component is O₂,as the reaction rate of O₂ is slower by about one digit and the effectby the main component is decreased as much, it would be possible tomeasure the impurities to a more accurate degree. In addition, theaddition of Ar makes the discharge of the atmospheric pressureionization mass spectrometer(11) more stable. Noises are reduced and thebase lines are also stabilized.

[0077] In addition, in case of using He as a main purified gas and Ar asa sub-purified gas, as the carrier gas could be He, the separatingcolumn(13) of the gas chromatography(10) gets more stable. Further, byadding He—Ar mixed gas into the outflowed gas, the H₂ impurities, whichhas been detected by mass number 9, can be detected by mass number 41 or81, and methane, which has been detected by mass number 15, can bedetected by mass number 16, and thus the sensitivity of the detectioncan be markedly improved.

[0078] That is, in case of adding He—Ar mixed gas, the generated He₄+can be destroyed and the background can be reduced, and it can bepossible to measure the methane of impurity by mass number 16. At thesame time, in case of using Ar in which case Ar+ or Ar₂+ becomes themain reactant, the detection of impurities H₂ could be possible by massnumber 41(ArH+) or mass number 81(Ar₂H+) with less noise, instead of thedetection by the mass number 9(He₂H+) with greater noise.

[0079] In case of using He as a main purified gas, and Ar as asub-purified gas, by switching the gases added to the outflowed gas withHe alone and He—Ar mixed gas, the impurities undetectable by adding amixed gas, such as N₂, Ne, Ar, etc., can be detected by He alone, whilethe scope of the subject components detectable can be enlarged and thesensitivity can be improved. Further, as the main purified gas of He canbe also used as a carrier gas, the labor and time for changing a carriercan be saved. Besides, the amount of the purified gas added to theoutflowed gas can be increased or decreased according to the kinds ofthe outflowed gases from the gas chromatography(10).

[0080] In case of using He and Ar, the ratio of Ar to He can beestablished freely. If the carrier gas is He, the concentration of theAr in the added gas is preferably settled to be 0-50%.

[0081] In addition, in case of measuring impurities of N₂ in Ar, forwhich many steps of work are needed up to now, the effect of N₂ impurityin H₂ can be ignored, by using a H₂-contained mixed purified gas as amixed gas to be added after being separated by gas chromatography,without introducing H₂ into the sample gas in the ion source part of theatmospheric pressure ionization mass spectrometer, such as He—H₂ mixedgas or Ar—H₂ mixed gas; and the N₂ impurity in Ar can be discriminatedand measured in detail, because the CO and N₂ can be separated by thegas chromatography. Moreover, as a H₂-added method, a H₂-contained mixedgas can be used as a carrier gas, or the improved sensitivity can beexpected by adding H₂ in measuring impurities, the sensitivity of whichcan be increased by using proton transfer reaction.

[0082] As the gas chromatography(10) or the atmospheric pressureionization mass spectrometer(11), the conventional ones can be used.Moreover, the scope of the main components of the sample and theimpurities to be analyzed are not limited specifically; not only thehighly purified gas such as O₂, N₂, H₂, Ar, He, Xe, Kr, and the like butalso the gases used as material for a semiconductor can be applied forthis analyzing method. Further, it is possible to add a purified gascomprising of 3 and more components, for example He, Ar and Ne, into theoutflowed gas.

[0083]FIG. 2 is a flow diagram for showing a second example of thepresent apparatus. In this analyzing apparatus, the gas outflowed intothe separated gas outflowing passage(21), is added by the main purifiedgas in a predetermined amount, and then added by the sub-purified gas inthe back. Like in the above case, the purified gas can be added into theoutflowed gas in a desired amount or in a desired mixed ratio, bycontrolling the flow of the both mass flow controllers(40,41) or byopening or shutting the valves(42,43).

[0084] As the other parts of the apparatus can be organized as in thefirst example, the same numbers will be given to the same constitutions,without repeating the detailed explanation (It will be the same in theother examples in the following).

[0085]FIG. 3 and FIG. 4 are flow diagrams showing the third and fourthexamples of the present apparatus. The examples show apparatuses bywhich the amount of the outflowed gas from the gas chromatography(11)can be increased to a minimum amount commonly required for theatmospheric pressure ionization mass spectrometer; or a minimum amountof the gas of the atmospheric pressure ionization mass spectrometer(11)can be decreased to the common amount of the outflowed gas from the gaschromatography(10); or the analysis can be performed by the atmosphericpressure ionization mass spectrometer(11) without adding the purifiedgas into the outflowed gas from the gas chromatography(10).

[0086] In the apparatus of the third example, the main purified gassupply system (57) and the sub-purified gas supply system (58), whichhave the pressure regulators(51,52), the purifiers(53,54) and mass flowcontrollers (55,56) respectively, are connected to the carrier gasintroduction passage(16) of the gas chromatography(10), so that one ofthe main purified gas and the sub-purified gas, or the mixed gases ofthe main purified gas and the sub-purified gas in an appropriate ratio,can be supplied as a carrier gas.

[0087] In the analyzing apparatus like the above, the most appropriatecarrier gas can be selected and used, depended on the kinds of the maincomponents of the sample gas or the impurities to be measured; and ahigh sensitive detection can be performed by selecting the gases whichare properly mixed according to the outflow timing of the subjectimpurities.

[0088] Further, in the apparatus of FIG. 4, the mixed gases in a fixedratio are filled into the gas cylinder(61), which is connected to thecarrier gas introduction passage(16) through the pressure regulator(62),purifier63) and mass flow controller(64). That is, if the subject samplegas to be analyzed is determined, the predetermined mixed gas is to besupplied from the gas cylinder(61), such that the apparatus can besimplified without diminishing of the accuracy of the analysis. Further,as a purifier(63) for refining the mixed gas, a purifier of getter typeis recommended.

EXAMPLE 1

[0089] The impurities contained in O₂ gas(H₂, methane, N₂, CO, CO₂) wasmeasured by using the apparatus of the FIG. 1. As the gaschromatography, Uni beads 1S was filled for measuring CO₂, and molecularsieve 13×S was filled for measuring others, into the separating columnwhich had a diameter of 4 mm and a length of 2 m and was made fromstainless steel. The obtained amount of the sample gas was 3 cc, and theflow amount of the carrier(main purified gas) of He was 42 cc/min.

[0090] As for the measurement of N₂, was performed, after He was addedinto the outflowed gas in 1000 cc/min. Then, after the amount of the gasadded to the outflowed gas was changed to 328 cc/min and Ar(sub-purifiedgas) was added to He, the measurements of H₂, methane, CO and CO₂ wereperformed while changing the Ar concentration in the range of 0-90%. Therelations between the Ar concentration and the intensities of the peaksof the respective impurities are shown in FIG. 5.

[0091] As apparent in FIG. 5, the intensities of the peaks in theAr-added case were higher than in the Ar-free case(Ar concentration=0),in all the cases.

[0092] In addition, FIG. 6 shows the measured peaks in the Ar-freecase(FIG. 6(a)) and in 5% Ar concentration case(FIG. 6(b)), for themeasurement of methane. It is recognized that if Ar is added, the noisesof the background were decreased and the base line became stabilized.FIG. 7 shows the calibration curve of methane in Ar-added cases, inwhich the obtained linearity was satisfactory and the accuracy was alsohigh enough.

[0093] The result was that in case of using He—Ar mixed gas(Arconcentration=5%), the detection limit value of H₂ in O₂ gas(S/N=2, thesame in the following) was 0.5 ppb, that of methane was 0.2 ppb and thatof CO was 0.3 ppb. In case of Ar-free, the detection limit value was 2ppb for H₂, 2 ppb for methane, lppb for CO and 0.5 ppb for N₂.

[0094] When measuring the impurities of the high purified O₂gas(≧99.99995%), the result was 1 ppb for H₂, less than 0.3 ppb formethane, 1.5 ppb for CO and 11 ppb for N₂. In this method in which thekind of the carrier was not changed and the gas added to the outflowedgas was switched with He alone and He—Ar mixed gas(Ar 5%), the analysisof impurities of 4 component was carried out successfully in a shortperiod of time of 30 minutes in a sub-ppb level.

EXAMPLE 2

[0095] The operation was carried out in almost the same condition withthe EXAMPLE 1, except that the subject impurities to be measured was CO,and the total amount of the gases added to the outflowed gas was 420cc/min. FIG. 8 shows the measured peaks for the Ar-free case (FIG. 8(a))and for the case of 3% of Ar concentration(FIG. 8(b)).

[0096] As apparent in FIG. 8, the peak in the case of 3% of Ar wasstronger and more stable in the base line compared with the cases ofAr-free. FIG. 9 shows a calibration curve for the case of 3% of Arconcentration. From the above, the detection limit of CO was 0.3 ppb.

EXAMPLE 3

[0097] The measurement of CO₂ in N₂ was carried out in the apparatus ofFIG. 1, by using Ar as a main purified gas and He as a sub-purified gas.The separating column had 4 mm of diameter and 1 m of length, and wasmade from stainless steel, being filled with Uni beads 1S. The obtainedamount of the sample gas was 4 cc, and the flow of the carrier gas was112 cc/min.

[0098] The measurement of CO₂ was carried out for the He-free case(FIG.10(a)) and for the case that the He concentration was 50% in thepurified gas(FIG. 10(b)), while the amount of the gases added to theoutflowed gas was 420 cc/min.

[0099] As apparent in FIG. 10, the intensity of the peak has becomeseveral times higher by adding He. Further, the detection limit of thecase of Ar alone was 0.6 ppb, while the detection limit was improved to0.2 ppb by adding Ar with 50% of He concentration into the outflowedgas.

EXAMPLE 4

[0100] The measurement of CO₂ in N₂ gas was carried out by using theapparatus showed in FIG. 3.

[0101] The separating column of the gas chromatography had 4 mm ofdiameter and 2 m of length and was made from stainless steel, beingfilled with molecular sieves 13×S. The obtained amount of the sample gaswas 5 cc, and the flow of the carrier gas was 112 cc/min.

[0102] A mixed purified gas(Ar concentration=about 2%), in which 2 cc ofAr was mixed to 110 cc of He, was used as a carrier gas. The temperatureof the separating column was 35° C. For the atmospheric pressureionization mass spectrometer, the slit diameter was set to be small andthe outlet valve was regulated, so that the inside of the ion sourcepart may not fall under the atmospheric pressure, even in the case thatthe purified gas is not supplemented to the outflowed gas from the gaschromatography.

[0103] As a result of the above, the detection limit of CO₂ was improvedto 0.5 ppb by adding Ar, compared with the case that the limit was 5 ppbwhen He alone was used as a carrier gas.

EXAMPLE 5

[0104] The measurement of N₂ in Ar was carried out by using theapparatus of FIG. 1. The separating column of the gas chromatography had4 mm of diameter and 2 m of length and was made from stainless steel,being filled with molecular sieve 13XS. The obtained amount of thesample gas was 3 cc, and the flow of the carrier gas (main purified gas)was 42 cc/min, in which He was used as a carrier gas.

[0105] He—H₂ mixed gas was used as the gas added to the outflowed gas,and the H₂ contents were set to be in the range of 0.05-0.4%. The addedamount of the mixed gas was determined so that the amount of the gasintroduced into the atmospheric pressure ionization mass spectrometer is500 cc/min.

[0106] As for analyzing N₂, the measured result of H₂ free case is shownin FIG. 11(a) and that of H₂-added case is shown in FIG. 11(b). Asapparent in both figures, N₂, which was never detected in H₂-free case,was detected as N₂.H+(mass number=29) by a proton transfer reactionresulted from the addition of H₂.

[0107] Moreover, as shown in FIG. 12, the calibration curve line of theH₂-added case was satisfactory. In the result of the measurement, whenthe mixed gas(He+0.05% of H₂) was used as an added gas, the detectionlimit of N₂ in Ar was 1 ppb.

[0108] In addition, when the amount of N₂ contained in Ar in a highpurified Ar gas cylinder was measured, 72 ppb of N₂ was detected.Further, when N₂ was measured after being purified through getter typepurifier, the N₂ amount was less than 1 ppb.

[0109] As explained in the above, the present method has made itpossible for the trace impurities in various gases to be detected in asensitive and detailed way, and to be analyzed in ppb-ppt levels in ashort period of time, without the requirement of the complicatedoperation and constitution of the apparatus.

1. A method for analyzing trace impurities in gases, the methodcomprising the steps of: separating main components and trace impuritiesfrom a sample gas conveyed by a carrier gas by using a gaschromatography; introducing the gases outflowed from said gaschromatography into an atmospheric pressure ionization massspectrometer; and analyzing the trace impurities, wherein a mixed gas isused as the carrier gas.
 2. A method for analyzing trace impurities ingases, the method comprising the steps of: separating main componentsand trace impurities from a sample gas conveyed by a carrier gas byusing a gas chromatography; adding purified gas into the gases outflowedfrom said gas chromatography; introducing the gases into an atmosphericpressure ionization mass spectrometer; and analyzing the traceimpurities, wherein a mixed gas is used as at least one of the carriergas and the purified gas.
 3. A method for analyzing trace impurities ingases according to claim 1 or 2, characterized in that the mixed gas isAr—He mixed gas.
 4. A method for analyzing trace impurities in gases,the method comprising the steps of: separating main components and traceimpurities from a sample gas conveyed by a carrier gas by using a gaschromatography; introducing the gases outflowed from said gaschromatography into an atmospheric pressure ionization massspectrometer; and analyzing the trace impurities, wherein a singlecomponent gas is used as the carrier gas, and purified gas, the kind ofwhich is different from the carrier gas, are added to the outflowed gas.5. A method according to claim 4, characterized in that if said carriergas is He, the added purified gas is one selected from the group of Aralone, He—Ar mixed gas, Ar—H₂ mixed gas and Ar—H₂ mixed gas, and if saidcarrier gas is Ar, the added purified gas is He alone, or He—Ar mixedgas.
 6. A method for analyzing trace impurities in gases, the methodcomprising the steps of: introducing a sample gas into a gaschromatography using He as a carrier gas; separating main components andtrace impurities from the sample gas; adding a purified gas of Ar aloneor Ar—He mixed gas into the gases outflowed from the gas chromatography;introducing the gases into an atmospheric pressure ionization massspectrometer; and analyzing the trace impurities, wherein the impurityof H₂ is detected by mass number 41 or 81, and the impurity of methaneis detected by mass number
 16. 7. A method for analyzing the traceimpurities in gases, the method comprising the steps of: introducing asample gas into a gas chromatography by using He as a carrier gas;separating main components and trace impurities from the sample gas;adding a purified gas into the gases outflowed from the gaschromatography; introducing the gases into an atmospheric pressureionization mass spectrometer; and analyzing the trace impurities,wherein at least two of i) He alone, ii) He—Ar mixed gas, iii) He—H₂mixed gas and iv) Ar—H₂ mixed gas, are selected, and switchably used assaid purified gases.
 8. A method according to one of claims 2,3,4,5,6and 7, characterized in that the added amount of the purified gases ischanged depending on the kind of the outflowed gas.
 9. A apparatus foranalyzing the trace impurities in gas, the apparatus comprising a gaschromatography for separating the main component and trace impuritiesfrom a sample gas conveyed by a carrier gas; an atmospheric pressureionization mass spectrometer connected to the back of the gaschromatography; and a purified gas adding passage for adding thepurified gases into the gas outflowed from the gas chromatography, thepurified gases adding passage being installed in a passage between thegas escaping part of the gas chromatography and the gas introductionpassage of the atmospheric pressure ionization mass spectrometer,wherein i) a passage for supplying a purified gas whose kind is the samewith the carrier gas, ii) a passage for supplying another purified gaswhose kind is different from the carrier gas, and iii) a mixed ratioregulating means for regulating the mixed ratio of both purified gasesare installed in the purified gas adding passage.
 10. An apparatusaccording to claim 9, wherein an added amount controlling means forregulating the added amount of the purified gases depending on the kindof the outflowed gas, is installed in the purified gas adding passage.